The supply voltage for a mains powered LED driver is typically generated from an external supply, or from a tertiary winding on a fly-back switching coil. When the device is first turned on, the usual approach is to initially charge the VCC supply from an input voltage sense resistor—through an internal diode—and then switch on an internal resistor to form a potential divider to allow the mains voltage measurement. Alternative techniques use an external Zener diode and bleed resistor as well as the supply from the tertiary winding. The power supply for the driver IC takes a significant number of external components however it is done.